In the explanations below, the term semiconductor light sources should be understood to mean, in particular, laser diodes and light emitting diodes (LEDs). A generic circuit arrangement is known from the prior art and is used for example for operating laser diodes in video projectors. A current-regulated step-up converter is used for setting the current provided to the laser diodes. Usually, 24 to 40 individual laser diodes are connected in series in three to five strings each having six to eight laser diodes. Each string is fed by a dedicated driver, wherein the clock frequency of the respective PWM controllers used in the driver is set by an ohmic resistor coupled to one of the inputs of the PWM controller. Said clock frequency determines the switching frequency of the associated converter switch.
However, a DLP projection system equipped with such a circuit arrangement exhibits undesired, visible fluctuations in the brightness of the projected image.
A laser diode string is usually used for realizing a color channel. A system having, for example, four semiconductor light source units accordingly realizes a four-channel projection system.